Researchers and clinicians have proposed that an infant shows at least some individual characteristics of behavior or temperament that will be relatively stable. Thomas, et al. (1968) have concluded that a particular cluster of temperament characteristics, including high activity level, negative mood, lack of adaptability, and other variables, defines a baby as a "difficult infant." The difficult infant, who makes his caregivers' work more demanding and less satisfying, was found by Thomas, et al. to be "high risk," i.e., more likely than other children to develop behavioral problems in early childhood. They give clinical anecdotes to demonstrate that is is not the temperament alone, but rather the parents' response to the child's temperament that determines the outcome. While it is provocative, this work has methodological flaws. It needs to be replicated and extended. The present study will attempt to do so. First, a well-validated parent-report instrument measuring degree of difficultness in 4-6 month infants will be developed. Second, using this questionnaire, samples of difficult, average, and easy babies will be drawn from all segments of the population and studied in comparison with one another. Since one resultant of the difficult infant syndrome, as well as a major determinant of later problems may be less-than-optimal mother-infant interaction, this relationship will be given special attention, both in naturalistic and structured settings, and in large- and fine-grained analysis. In addition, by use of techniques such as sequence analysis the project will attempt to study direction of effects in the relationship. Third, the children's characteristics and developmental progress will be monitored, for some children until age 2, to see whether difficultness and poor mother-infant relationships do indeed predict later problems. The study will take a multivariate, multi-method approach and have several independent replications of findings, using a modified version of an accelerated longitudinal design.